Sand fracturing through coiled tubing and through snubbing units has allowed the development of new trends in well stimulation. The ability to perforate multiple zones in a single well and then fracture each zone independently has increased access to more potential reserves.
The fracturing program starts at the lowest zone in the well bore. The term fracturing refers to the use of fluids and proppants utilized for injection at high pressure into oil or gas wells, to fracture the geological formations surrounding the well, and thereby increasing their productivity. This permits more efficient flow of hydrocarbons and accelerates access to the reserves.
The purpose of the fracturing fluid is two fold: first to transmit energy generated at surface down the well bore to hydraulically create a fracture within reservoir rock, and secondly, to transport a proppant agent (usually sand) from surface to the reservoir to ensure conductivity generated by the fracture is preserved.
A hydraulic fracturing treatment typically consists of three main stages. Initially a “Pad” stage is pumped to initiate the fracture and create width for the stages to follow. The fluid pumped through this initial stage consists of the fracturing fluid without proppants. After a sufficient volume of Pad has been pumped, proppant is added to the fracturing fluid to form the “Slurry” stage. Concentrations of the proppant (sand, resin-coated sand, or ceramics) typically are kept low at the beginning and slowly ramped up to maximum values, which vary as a function of depth, fracturing pressures and reservoir type. An optimization process utilizing numerical and analytical simulation models can be used to determine the amount of proppant that is pumped, as is known in the art. Once the appropriate volume of proppant has been mixed by the blender and pumped down the well bore, a “Flush” stage, consisting of more fracturing fluid, is used to displace the slurry stage to the perforations.
Treatment design is based on several parameters that include, but are not limited to, reservoir permeability, pressure, depth, temperature and reservoir fluid type. Fracture fluid viscosity, down-hole injection rates, proppant size and type, proppant volume and concentrations are all important aspects of the final stimulation program. As is well known in the art, engineering modelling tools, together with previous field experience gained in each area, are used in a combined approach to formulate the best possible stimulation design for the reservoir.
A desirable feature in a fracturing fluid is variable viscosity. That is, fluids will frequently contain additives that can be selectively added, chemically or physically, to increase or decrease the viscosity of the fluid. The reason a high viscosity is desired is for the transport of proppant down the well bore and into the fracture, such as sand granules into a fractured formation to prevent the fracture from completely closing in the formation. The proppant ensures that the conductivity of the fracture is maintained. Afterwards, it is desirable to lower the viscosity of the fluid, so that it will flow out of the fracture into the well bore and to surface, allowing the flow of hydrocarbons to begin or resume.
Prior to commencement of the fracturing treatment, the straddle packer is placed across the lowest perforated interval and that zone is then fractured. Generally, a straddle packer comprises a pair of vertically spaced apart seals mounted on a tubular barrel that has an orifice to allow the fracturing fluid pumped through the barrel's interior to escape into the annulus between the barrel and the well casing. The pressure of the fluid expands the seals into sealing contact with the casing's inner wall so that the fluid then diverts itself through the perforations in the casing into the targeted formation. The seals are set sufficiently far apart to straddle the width of the zone to be fractured.
After treatment of the lowest zone, the tool is moved up the casing to the next perforated interval and this zone is then fractured. This operation is repeated for all the perforated intervals.
Particularly if the fracturing fluids have been energized, that is, co-mingled with a pressurized gas such as C02 or N2, it becomes extremely important to complete all the zones quickly and then allow the well to begin flowing back from the co-mingled zones for recovery of injected fluids.
Current isolation tools work effectively at isolating the zone and fracturing once down the well bore. However, when fracturing multiple zones in the well bore, and when the pressure of a previously treated lower zone exceeds the resistance of the tool's lower sealing member, fluid with sand will flow past the lower sealing member, collapsing it, and possibly even flowing into the tool body. This can prevent the tool from moving up the well bore, seating at the next interval or sealing the next set of perforations. These consequences can all create serious job problems and/or failures.